1. Two extruder screw shafts on an HDPE plant in Indonesia failed at the kneading/mixing zone where high pressure and bending moment occur. 2. Visual examination found the failures were mainly due to fatigue bending with some torsion fatigue, initiated by multiple crack origins with only one developing into a large propagation area. 3. The root cause was fatigue failure from concentrated loading on the hexagonal shaft surface during overloading, likely due to a non-zero clearance fit between the shaft and screw that unevenly distributed torque.